provides information on changes in rock type near the wellbore from changes in measured gamma radiation

Stratigraphic/Structural:

using multiple gamma logs over an area, the depth to the sandstone and shale layers can be correlated over larger areas

Hydrological:

Thermal:

Cost Information

Low-End Estimate (USD):

0.2525 centUSD 2.5e-4 kUSD 2.5e-7 MUSD 2.5e-10 TUSD / foot

Median Estimate (USD):

0.3838 centUSD 3.8e-4 kUSD 3.8e-7 MUSD 3.8e-10 TUSD / foot

High-End Estimate (USD):

0.7575 centUSD 7.5e-4 kUSD 7.5e-7 MUSD 7.5e-10 TUSD / foot

Time Required

Low-End Estimate:

0.35 days9.582478e-4 years 8.4 hours 0.05 weeks 0.0115 months / job

Median Estimate:

0.69 days0.00189 years 16.56 hours 0.0986 weeks 0.0227 months / job

High-End Estimate:

0.69 days0.00189 years 16.56 hours 0.0986 weeks 0.0227 months / job

Gamma Log:

Gamma logging is a method of measuring naturally occurring gamma radiation to characterize the rock or sediment in a borehole or drill hole. It is a wireline logging method used in mining, mineral exploration, water-well drilling, for formation evaluation in oil and gas well drilling and for other related purposes. Different types of rock emit different amounts and different spectra of natural gamma radiation.

Natural-gamma ray logging records the gamma radiation emitted from rocks and minerals within open or cased well bores that are either liquid or air filled. Potassium-40, Uranium-238 (series in equibrium) and Thorium-232 (series in equibrium) are the most common naturally occurring radioactive elements measured during natural-gamma ray logging. The natural-gamma ray log is a passive measurement, that is it measures the radiation emitted.

The most common and widley used gamma ray sensor uses a thallium doped sodium iodine crystal to detect the radiation. The detector does not discriminate the radiation emited from different mineral/rock sources.

Acidic igneous rocks (i.e., granites, ryolites, calc-alkine schists, etc.)have higher concentrations of Potassium-40(i.e., feldspars and micas) and radioneuclides than basic igneous rocks (i.e., bassalt, andesite, etc.). This is due to a higher concentration of Potassium-40 in feldspars and micas, which are easily altered to clay. Sandstones and limestones generally do not contain appreciable percentages of clay or radioactive minerals. This relationship provides a common and inexpensive method to distinguish lithologies within a well bore and stratigraphic correlation between well bores.

Use in Geothermal Exploration

The concentration of radioactive materials that emit gamma rays varies among different rock types. Acidic igneous and metamorphic rocks have a higher concentration of radioactive isotopes that the basic basaltic and andesitic rock types.

The gamma ray log can be analyzed and corrected for well bore effects and the differences in radioactivity of the igneous and metamorphic rock types. The presence of altered zones, cap rocks and shale layers, can also be identified.

Typically run as one logging method in a standard suite of bore-hole logs.

Environmental Mitigation Measures

Measures associated with well drilling and well logging operations.

Data Access and Acquisition

A detector located within a wireline probe is lowered into the well and typically measures the gamma ray intensity when raised from the bottom. The detector can either be a scitillation counter (Thallum doped sodium iodice crystal) or Geiger-Muller tubes record and transmit the total radioactivity that is displayed as the gamma ray log.

Best Practices

If multiple wells are drilled within a geothermal field, correlating the gamma radiation data with depth can help to correlate lithologies between wells. Natural faulting of the system can alter the depth to the various layers between the wells.

Potential Pitfalls

Mostly shales and sandstones account for most of the gamma radiation. However, there are some other rocks such as some carbonates or feldspar rich rock types that produce radiation and could hinder the accurate detection of the sands and shales.